1. Field of the Invention
The invention relates to calibrating a device under test (DUT) to communicate in wireless networks, and more particularly, to a method for calibrating a DUT in order to map signal strength data measured for the DUT to reference signal strength data measured for a reference device.
2. Description of the Prior Art
In wireless telecommunications, a received signal strength indicator (RSSI) is a measurement of the power level in a received radio signal. The RSSI is a key measurement that is used in a variety of applications such as inter-access point handover strategies, rate adaptation, and location sensing. In general, the higher the RSSI value of a received signal is, the more likely it is that the received signal will be correctly received. Many applications are based on the accuracy of the RSSI.
Please refer to FIG. 1. FIG. 1 illustrates a conventional operating environment in which communication units (CU) 20, 22, 24, 26 attempt to wirelessly communicate with an access point (AP) 10. To communicate with each other through the access point 10, communication units 20, 22, 24, 26 should first join a basic service set (BSS) mandated by the access point 10.
The relationship between RSSI and dependent variables in the applications can be predetermined if RSSI is calibrated for the communication units 20, 22, 24, 26. End users can use these applications without an additional training procedure. This will increase the usefulness and practicability of the applications.
Please refer to FIG. 2. FIG. 2 is a chart showing a relationship between transmission power level and RSSI according to the prior art. The existing calibration process is based on the relationship between the transmission power level and the received RSSI value of signals received at that corresponding transmission power level. In FIG. 2 the x-axis (horizontal axis) represents the transmission power level measured in dBm and the y-axis (vertical axis) represents the RSSI value of the received signal in a reference communication unit.
Wireless signals are transmitted to the reference communication unit at a variety of different power levels, and the corresponding RSSI values are obtained. Using the (power level, RSSI value) pairs, a nominal curve 30 of the reference communication unit is constructed, as shown in FIG. 2.
In the typical calibration process for a device under test (DUT), an unadjusted curve of the DUT is obtained by applying the same procedure of obtaining (power level, RSSI value) pairs for several different power levels. Now there are two curves existing in the same coordination system, the nominal curve 30 taken from the reference communication unit, and the unadjusted curve taken from the DUT, and each curve shows RSSI values versus the corresponding power level values. The calibration process involves trying to match the unadjusted curve for the DUT with the nominal curve 30 for the reference communication unit by changing the offset and slope, etc. of the unadjusted curve in order to produce an adjusted curve that matches the nominal curve 30 as closely as possible.
However, due to differences between the shapes of the nominal curve 30 and the unadjusted curve, the adjusted curve cannot always be well matched with the nominal curve 30 by simply changing the offset or slope of the unadjusted curve. Therefore, the RSSI values corresponding to some transmission power levels of the adjusted curve cannot always be well calibrated due to the differences in the (power level, RSSI value) pairs between the adjusted curve and the nominal curve 30. As a result, there is a need for an improved calibration method to more accurately calibrate a DUT in order to map signal strength data measured for the DUT to reference signal strength data measured for a reference communication unit for given transmission power levels.